Charged droplet production unassisted or pneumatic nebulization assisted Electrospray (ES) requires oxidation of species (positive ion polarity ES) or reduction of species (negative ion polarity) at conductive surfaces in the sample solution flow path. When a metal electrospray needle tip is used that is electrically connected to a voltage or ground potential, such oxidation or reduction reactions (redox) reactions occur on the inside surface of the metal electrospray needle during electrospray ionization. If a dielectric electrospray tip is used in electrospray ionization, redox reactions occur on an electrically conductive metal surface contacting the sample solution along the sample solution flow path. This conductive surface typically may by a stainless steel union connected to a fused silica electrospray tip. The electrospray sample solution flow path forms one half cell of an electrochemical or voltaic cell. The second half of the electrochemical cell formed in electrospray operates in the gas phase. Consequently, operating rules that explain or predict the behavior of liquid to liquid electrochemical cells may be applied to explain a portion of the processes occurring in electrospray ionization. The electrolyte aids in promoting redox reactions occurring at electrode surfaces immersed in liquid in electrochemical cells. The electrolyte not only plays a role in the initial redox reactions required to form single polarity charged liquid droplets but also fundamentally affects the production of sample related ions from rapidly evaporating liquid droplets and their subsequent transport through the gas phase into vacuum. Additional charge exchange reactions can occur with sample species in the gas phase. The mechanism by which the electrolyte affects liquid and gas phase ionization of analyte species is not clear.
The type and concentration of electrolyte species affects ES ionization efficiency. The electrolyte type and concentration and sample solution composition will affect the dielectric constant, conductivity and pH of the sample solution. The relative voltage applied between the Electrospray tip and counter electrodes, the effective radius of curvature of the electrospray tip and shape of the emerging fluid surface determine the effective electric field strength at the electrospray needle tip. The strength of the applied electric field is generally set just below the onset of gas phase breakdown or corona discharge in electrospray ionization. With an effective upper bound on the electric field that is applied at the electrospray tip during electrospray operation, the electrospray total ion current is determined by the solution properties as well as the placement of the conductive surface along the sample solution flow path. The effective conductivity of the sample solution between the nearest electrically conductive surface in contact with the sample solution and the electrospray tip plays a significant in determining the electrospray total ion current. It has been found with studies using electrospray membrane probes that the ESMS analyte signal can vary significantly with electrospray total ion current. A description of the electrospray membrane probe is given in U.S. patent application Ser. Nos. 11/132,953 and 60/840,095 and incorporated herein by reference.
ES signal is enhanced when specific organic acid species such as acetic and formic acids are added to organic and aqueous solvents. Conversely, ES signal is reduced when inorganic acids such as hydrochloric or trifluoroacetic acid are added to electrospray sample solutions. Although mechanisms underlying variation in electrospray ionization efficiency due to different electrolyte counter ion species have been proposed, explanations of these root modulators underlying electrospray ionization processes remain speculative. Conventional electrolytes added to sample solutions in electrospray ionization are generally selected to maximize electrospray MS analyte ion signal. Alternatively, electrolyte species and concentrations are selected to serve as a reasonable compromise to optimize upstream sample preparation or separation system performance and downstream electrospray performance. Trifluoroacetic acid may be added to a sample solution to improve a reverse phase gradient liquid chromatography sample separation but its presence will reduce the electrospray MS signal significantly compared with electro spraying with an organic electrolyte such as formic or acetic acid added to the sample solution. Generally for polar analyte species, the highest electrospray MS signal will be achieved using a polar organic solvent such as methanol in water with acetic or formic acid added as the electrolyte. Typically, a 30:70 to 50:50 methanol to water ratio is run with acetic or formic acid concentrations ranging from 0.1% to over 1%. Running non polar solvents, such as acetonitrile, with water will reduce the ESMS signal for polar compounds and adding inorganic acid will reduce ESMS signal compared to the signal achieved using a polar organic solvent in water with acetic or formic acid. Several species of acids bases and salts have been used at different concentrations and in different solvent compositions as electrolyte species in electrospray ionization to maximize ESMS analyte species. For some less polar analyte samples that do not dissolve in aqueous solutions, higher ESMS signal is achieved running the sample in pure acetonitrile with an electrolyte. For compounds such as carbohydrates with low or no proton affinity, adding a salt electrolyte may product higher ESMS signal.
The invention comprises using a new set of electrolyte species in electrospray ionization to improve the electrospray ionization efficiency of analyte species compared with ES ionization efficiency achieved with conventional electrolyte species used and reported for electrospray ionization. Electrospraying with the new electrolyte species increases ESMS analyte signal amplitude by a factor of two to ten for certain analyte species compared to the highest ESMS signal achieved using acetic or formic acids for these sample species. ESMS signal enhancements have been achieved whether the new electrolytes are added directly to the sample solution or added to the second solution of an electrospray membrane probe. When convention acid or salt electrolytes added to the sample solution are electrosprayed in positive polarity mode, the anion from these electrolytes does not readily appear in the positive ion spectrum. As expected, the anion of these electrolytes does appear in the negative ion polarity ESMS spectrum. One distinguishing characteristic of the new electrolytes comprising the invention is that a characteristic protonated or deprotonated parent related ion from the electrolyte species appears in both positive and negative polarity spectrum acquired using electrospray ionization. The positive polarity electrolyte ion appearing in the positive polarity electrospray mass spectrum is the (M+H)+ species with the (M−H)− species appearing in the negative polarity electrospray mass spectrum.
An alternative embodiment of the invention is the addition of certain electrolytes into the second solution of an electrospray membrane probe to enhance the ESMS signal amplitude of certain analyte species added to the sample solution flow. The alternative embodiment of the invention increases the ESMS signal compared to the ESMS signal amplitude achieved when the same electrolyte species are added directly to the sample solution during electrospray ionization.